1. Field of the Invention
The present invention relates to a color image reading apparatus. More particularly, the invention relates to a color image reading apparatus suitable for color scanners, color facsimile devices, or the like, which can read information of color image on manuscript at high precision, making use of color separation means comprised of a volume hologram and an optical interference film, and reading means in which three line sensors are provided on a single substrate.
2. Related Background Art
There have been proposed various reading apparatus in which color image information on manuscript is guided through an optical system to form images thereof on line sensors such as CCD and the color image information is digitally read using output signals from the line sensors.
For example, FIG. 1 is a schematic drawing of a conventional color image reading apparatus. In FIG. 1, a beam from color image on manuscript 1 is converged by an image-forming lens 20; in forming images on line sensors described below the beam is guided through a 3P prism 21 to be color separated into three colors, for example red (R), green (G) and blue (B); and the thus separated color beams are guided onto line sensors 22, 23, 24 each composed of CCD for example. Each color image formed on line sensor 22, 23, 24 is line-scanned to effect reading for each color light.
FIG. 2 is a schematic drawing to show the major part of a color image reading apparatus proposed in Japanese Laid-open Patent Application No. 62-234106.
In FIG. 2, a beam from color image on manuscript 1 is converged by an image-forming lens 25 and in forming images on line sensors described below, the beam is separated into three beams corresponding to the three colors through two beam splitters 26, 27 for color separation with selective transmission films having dichroism. Color images based on the three color light beams are formed on respective line sensors in so-called monolithic 3-line sensor 28, in which three line sensors 28a, 28b, 28c are provided on a single substrate as shown in FIG. 3. This enables reading of each color light with line scan on the color image. In the arrangement shown in FIG. 3, distance S.sub.1 or S.sub.2 between line sensors is about 0.1 to 0.2 mm, and the size of each unit sensor element W1.times.W2 is about 7 .mu.m.times.7 .mu.m or 10 .mu.m.times.10 .mu.m.
The color image reading apparatus shown in FIG. 1 needs the three independent line sensors, must be made at high precision, and requires the 3P prism which is difficult to produce, which caused the following problems. The entire apparatus becomes complicated and expensive. Further, assembly adjustment is troublesome, because three independent aligning adjustments are necessary between the image-forming beams and the line sensors.
Supposing the thickness of beam splitter 26, 27 is X in the color image reading apparatus shown in FIG. 2, the distance between two lines of line sensors is 2.sqroot.2X.
If a preferable distance is about 0.1 to 0.2 mm between the lines of line sensors in respect of production, the thickness X of beam splitter 26, 27 should be about 35 to 70 .mu.m.
Generally, it is very difficult to make such a thin beam splitter as keeping excellent optical planeness. Thus, if such thin beam splitters were used, the optical property of color images formed on line sensors would be degraded.
Meanwhile, it is generally normal that the two line sensors 28a, 28c are separated at equal distance on either side of the central line sensor 28b in the monolithic 3-line sensor and that the distance between the central line sensor 28b and either one of the other line sensors 28a, 28c is an integral multiple of pixel size in the sub-scanning direction (W2 in FIG. 3). The reason is as follows. As seen from FIG. 4, in case that an image is read by the above monolithic 3-line sensor, using only a normal image-forming lens 25, three different positions 51a, 51b, 51c are simultaneously read on the manuscript by the three line sensors 28a, 28b, 28c, as shown in FIG. 4. Therefore, three signal components of three color light (R, G, B) cannot be simultaneously read at a certain position. Three components must be set together to be combined with each other after all components are read.
To realize it, the distance S.sub.1, S.sub.2 between line sensors is set to an integral multiple of each pixel size W2 and redundant line memories are provided corresponding to the integral multiple. Then G signal and R signal are delayed relative to B signal for example, relatively easily to obtain a combined signal of three colors. This is the reason for the above integral multiple. However, a drawback is that a plurality of expensive line memories must be provided as the redundant line memories corresponding to the distance between lines.